This invention relates to burner systems for burning granules, pellets or similarly sized solid biomass heating fuel. It relates more particularly to a burner system that incorporates a close-coupled pyrolysis gasifier.
Pellet burners or stoves are in wide use. Such burners typically use a biomass fuel in the form of small pellets of about 6 mm in diameter and about 25 mm in length made from waste from wood processing industries. Such pellets typically have a heat value of approximately 8,500 BTU""s per pound (19.8 MJ/kg).
Pellet burners are generally considered to have advantages over conventional stick wood burning stoves. One advantage is that substantially less emissions are produced by pellet burners than by wood burners. Also, most pellet burning stoves meet environmental emission standards. Pellet burning stoves typically achieve a higher efficiency than conventional wood stoves and some rather extravagant claims have been made as to this efficiency.
In Wood Energy Institute News, Jan. 19, 1992, Daniel Malcon explained that a problem with pellet stoves lies in heat transfer efficiency, i.e. how much of the potential heat created by the fire is actually delivered into the home. Pellet stoves typically operate with high excess air, e.g. an air:fuel ratio greater than 35:1, and as this air goes through the stove and out the chimney, it evacuates heat that would otherwise be transferred to the home. Thus, even if flue temperatures are relatively low, the volume of air is so great that much of the heat is lost out the flue. As a result, typical existing pellet stoves have an average overall efficiency, i.e. the percentage of potential heat delivered into the room, in the order of about 50%.
Another problem that has been encountered with pellet stoves is that they tend to be very sensitive to the quality of the pellets being fed. Thus, pellets having a high ash content tend to produce slag upon combustion and form clinkers which further reduce efficiency and increase maintenance requirements. Typical current pellet stoves cannot operate with pellets containing more than about 1 to 3% ash.
Miller et al., U.S. Pat. No. 4,782,765, issued Nov. 8, 1988, describes a pellet burner which includes a retort means in which the pellets are heated to approximately 590xc2x0 C. and combustion gases rising from these heated pellets are combusted while reaching temperatures in the order of 1200 to 1370xc2x0 C. This system utilizes a bottom auger feed for the pellets with combustion air being supplied from a central perforated tube. Because of the manner in which the air contacts the bed of pellets being fed upwardly from the bottom, the burner can only operate with high excess air and is not capable of variable control.
Another pellet stove is described in Beierle et al., U.S. Pat. No. 4,738,205, issued Apr. 19, 1998. In that system, the pellets are fed into a gasifier and the gasifier is operated to produce carbon and fuel gas. In this design, the produced fuel gas and carbon are drawn down from the bottom of the gasifier and by means of a blower are moved through an exit nozzle and a tubular pipe into a burner section. This is a complicated and difficult system to operate and also can operate only with high excess air.
It is an object of the present invention to provide a simple and inexpensive pellet burner that is capable of operating with pellets containing as much as 10% ash. This would greatly expand the scope of biomass materials that could be used for pellets.
It is a further object of the present invention to provide a very simple design of pyrolysis/gasification chamber which is capable of combusting the high ash pellets without the formation of fusion products such as slag and clinkers.
It is a still further object of the present invention to provide a pellet burner capable of consistently operating at high overall efficiencies in the order of 85%.
Yet another object of the invention is to provide a pellet burner having a controlled variable combustion air flow while operating at a much lower air:fuel ratio than conventional pellet stoves.
The present invention in its broadest aspect relates to a burner system for burning pellets or granules of solid biomass heating fuel comprising an air tight fire chamber having an exhaust outlet in an upper region thereof and a burner in the bottom thereof for converting the solid fuel to fuel gases and ash in the presence of a limited air supply and combusting the gases formed. The burner comprises upright co-axial inner and outer cylindrical walls providing a confined central gasification/combustion chamber surrounded by an annular combustion air manifold. The annular combustion air manifold is closed at the top and bottom with an air inlet in the outer wall and a plurality of spaced apertures arranged in upward rows in the inner cylindrical wall providing air inlets from the combustion air manifold to the combustion zone of the chamber. A grate at the bottom of the gasification/combustion chamber holds the solid fuel while allowing ash to pass through the grate. An air tight ash-receiving chamber is provided below the gasification/combustion chamber and fan means are provided for producing a negative pressure in the gasification/combustion chamber to thereby draw combustion air into and upwardly through the chamber. In this way, solid fuel fed to the grate at the bottom of the gasification/combustion chamber is pyrolyzed in the presence of a limited air supply to form combustion gases which are drawn upwardly through the gasification/combustion chamber while contacting heated combustion air entering through the spaced air inlets.
This is operated as an air-tight system with the only exterior connections being an inlet for combustion air and an exhaust gas outlet.
The location and size of the combustion air inlet holes is an important feature of this invention and these are spaced in a upward path along the length of the inner cylindrical wall. They are preferably arranged in the form of at least two spiral paths extending up the wall with the most preferred system being a pair of spiral arrays of inlet holes arranged as mirror image spiral paths on opposite sides of the inner cylindrical wall.
The pellets are fed into the gasification/combustion chamber either from a top feeder or a bottom feeder and rest on the grate at the bottom. In that location, the pellets receive a very limited combustion air supply sufficient only to raise the temperature in the pellets to about 480 to 590xc2x0 C., i.e. below melting temperatures of the minerals that may be present. At this temperature, gases are released and rise within the combustion zone where they are super-combusted with controlled flow of hot combustion air. The spiral array of combustion air inlet holes is located sufficiently far above the pellets being gasified on the grate so as to induce by negative pressure the gases and flames away from the grate area, thereby keeping the pellets on the grate below the temperature at which slag and clinkers are formed. The duel spiral array of air inlet holes arranged in the above manner cause the introduction of super heated combustion air into the chamber with a vertically swirling and turbulent motion which results in very complete burning of the gases emanating from the solid fuel below. This complete burning is achieved at temperatures in the order of 1200 to 1370xc2x0 C. without the need of a large excess of air and this means that the flow of exhaust gas is substantially reduced compared to conventional pellet stove appliances. The burner of this invention operates very efficiently at air:fuel ratios of less than 8:1 and an excess air level of no more than 100%.
An important feature of the burner arrangement of the invention is that because of the low combustion air flows and the manner of introducing the combustion air, the system produces very low particulate emission levels and little airborne fly ash even with the use of high ash fuels. The combustion air flow can also be varied and closely controlled. This means that it is possible to operate the burner over a wide range of heat outputs. For instance, a residential pellet stove using the burner system of this invention can be operated at heat outputs ranging from 4,415 to 28,900 BTU/hr (10.3 to 67.2 MJ/kg) by varying the fuel flow and combustion air flow. In other words, the combustion air flow is varied in accordance with the fuel input for optimum operation.
According to a preferred feature, the system includes an oxygen sensor so as to substantially increase the overall steady state efficiency level by controlling the combustion air level to pellet fuel quantity ratio and rate of burn within the chamber. The amount of combustion air to the zone in which the gases are burned is regulated by the oxygen sensor in proportion to the amount of pellet fuel being combusted. The oxygen sensor employs a feedback loop to a main control system for the burner, which controls the fan motor speed supplying the induced combustion air being drawn through the combustion zone. The oxygen sensor measures oxygen concentrations between lean (any oxygen in the exhaust system not being utilized to combust pellets) to rich in oxygen (when there is not enough oxygen for the amount of fuel present in the combustion zone). The oxygen sensor balances the level of air to fuel so as to maintain an optimum rate of burn, which in turn effects the efficiency level of the combustion process. This oxygen sensor is conveniently located in the exhaust system to monitor the exhaust gases.
The pellets are preferably held in a storage container in the form of a hopper with a fuel auger system lifting pellets from the bottom of the hopper up to a feed chute for directing the pellets into the top end of the gasification/combustion chamber. The rate of feeding of the pellets is controlled by the rotational speed of the auger. With this top feeding system, there is the advantage that the pellets falling down through the combustion zone are preheated so that they quickly reach the pyrolysis temperature of about 480 to 590xc2x0 C. Also, if the fuel has high moisture, much of this can be removed as the pellets fall through the combustion zone.
It is also possible to feed the pellets directly into the bottom of the burner. For this arrangement, a generally horizontal auger moves the pellets from a storage hopper directly onto the grate beneath the gasification/combustion chamber.
The burner system of this invention is capable of efficiently burning solid biomass heating fuel containing as high as 10% ash without the formation of fusion products such as slag and clinkers. In a typical prior pellet stove, problems are usually encountered with pellets containing more than about 1 to 3% ash. Higher ash contents have tended to result in fusion products forming on the grate and also obstructing air holes, necessitating a shutdown of the prior pellet stoves to remove this interfering material.
The solid biomass fuel used in the burner system of this invention is typically processed fuels of increased bulk density. These may be formed from many different materials, such as pellets from typical lignocellulose materials, e.g. wood chips or sawdust. They may also be formed from agricultural residues such as cherry pits, olive pits, etc. which may be used without modification. It is also possible to produce pellets from cheap and specially grown various forms of grasses and vegetable matters as feedstock. It will be understood that wherever the term xe2x80x9cpelletsxe2x80x9d is used throughout this text, it is referring to any type of processed biomass fuel of increased bulk density.
The grate for supporting the pellets is preferably formed of a plurality of three sided metal rods having a predetermined distance between them. Thus, they are arranged so that each rod has a central peak and a pair of downwardly inclined faces. The space between adjacent rods is slightly less than the diameter of the fresh pellets being used. When the pellets become carbonized, they are small enough to drop through between the rods of the grate. Loose ash also slides down the inclined faces. According to a particularly preferred feature, a pair of grates are used, one below the other. When the pair of grates are used, the lower grate tends to hold material that has dropped through the upper grate which is not fully carbonized so that the burning may be completed on the second grate. Whether one or a pair of grates is used, a vibrating means may be used to assist the substantially carbonized biomass fuel to fall through the grate(s).
The burner system of this invention typically includes two blowers. A combustion air blower is provided preferably in the exhaust system for drawing combustion air in through the burner and out through the exhaust. This provides a negative pressure in the gasification/combustion chamber. A convection blower circulates air around the fire chamber and into a room for heating. The blowers and the fuel auger may be powered by either AC or DC motors. Because of the small amount of energy required, it is particularly desirable to use DC motors. Not only is such a system very safe, but it provides a further advantage that the system can be powered by a 12 volt battery in the event of an electrical power failure.